TPS51218 Datasheet, PDF (14/27 Pages) Texas Instruments – HIGH PERFORMANCE, SINGLE SYNCHRONOUS STEP-DOWN CONTROLLER FOR NOTEBOOK POWER SUPPLY

TPS51218
SLUS935A – MAY 2009 – REVISED JUNE 2009 ............................................................................................................................................................... www.ti.com
LIGHT LOAD CONDITION IN AUTO-SKIP OPERATION
With RF pin pulled down to low via R_RF, the TPS51218 automatically reduces switching frequency at light load
conditions to maintain high efficiency. As the output current decreases from heavy load condition, the inductor
current is also reduced and eventually comes to the point that its rippled valley touches zero level, which is the
boundary between continuous conduction and discontinuous conduction modes. The rectifying MOSFET is
turned off when this zero inductor current is detected. As the load current further decreases, the converter runs in
to discontinuous conduction mode. The on-time is kept almost the same as it was in the continuous conduction
mode so that it takes longer time to discharge the output capacitor with smaller load current to the level of the
reference voltage. The transition point to the light load operation I_O(LL) (i.e., the threshold between continuous and
discontinuous conduction mode) can be calculated in Equation 3.
I_O(LL)
=
1
2 ´ L ´ f_SW
(V_IN
´
- V_OUT ) ´ V_OUT
V_IN
(3)
where
• f_SW is the PWM switching frequency
Switching frequency versus output current in the light load condition is a function of L, V_IN and V_OUT, but it
decreases almost proportional to the output current from the I_O(LL) given in Equation 3. For example, it is 58 kHz
at I_O(LL)/5 if the frequency setting is 290 kHz.
ADAPTIVE ZERO CROSSING
The TPS51218 has an adaptive zero crossing circuit which performs optimization of the zero inductor current
detection at skip mode operation. This function pursues ideal low-side MOSFET turning off timing and
compensates inherent offset voltage of the ZC comparator and delay time of the ZC detection circuit. It prevents
SW-node swing-up caused by too late detection and minimizes diode conduction period caused by too early
detection. As a result, better light load efficiency is delivered.
FORCED CONTINUOUS CONDUCTION MODE
When the RF pin is tied high, the controller keeps continuous conduction mode (CCM) in light load condition. In
this mode, switching frequency is kept almost constant over the entire load range which is suitable for
applications need tight control of the switching frequency at a cost of lower efficiency. To set the switching
frequency to be the same as Auto-skip mode, it is recommended to connect R_RF to PGOOD. In this way, RF is
tied low prior to soft-start operation to set frequency and tied high after powergood indicates high.
OUTPUT DISCHARGE CONTROL
When EN is low, the TPS51218 discharges the output capacitor using internal MOSFET connected between SW
and GND while high-side and low-side MOSFETs are kept off. The current capability of this MOSFET is limited to
discharge slowly.
LOW-SIDE DRIVER
The low-side driver is designed to drive high current low R_DS(on) N-channel MOSFET(s). The drive capability is
represented by its internal resistance, which are 1.0Ω for V5IN to DRVL and 0.5Ω for DRVL to GND. A dead time
to prevent shoot through is internally generated between high-side MOSFET off to low-side MOSFET on, and
low-side MOSFET off to high-side MOSFET on. 5-V bias voltage is delivered from V5IN supply. The
instantaneous drive current is supplied by an input capacitor connected between V5IN and GND. The average
drive current is equal to the gate charge at Vgs=5V times switching frequency. This gate drive current as well as
the high-side gate drive current times 5V makes the driving power which need to be dissipated from TPS51218
package.
HIGH-SIDE DRIVER
The high-side driver is designed to drive high current, low R_DS(on) N-channel MOSFET(s). When configured as a
floating driver, 5 V of bias voltage is delivered from V5IN supply. The average drive current is also equal to the
gate charge at V_GS=5V times switching frequency. The instantaneous drive current is supplied by the flying
capacitor between VBST and SW pins. The drive capability is represented by its internal resistance, which are
1.5 Ω for VBST to DRVH and 0.7 Ω for DRVH to SW.
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